This study examined the psychometric properties of the Revised Edition of the School Observation Coding System (REDSOCS). Participants were 68 children ages 3 to 6 who completed parent-child interaction therapy for Oppositional Defiant Disorder as part of a larger efficacy trial. Interobserver reliability on REDSOCS categories was moderate to high, with percent agreement ranging from 47% to 90% (M = 67%) and Cohen's kappa coefficients ranging from 0.69 to 0.95 (M = 0.82). Convergent validity of the REDSOCS categories was supported by significant correlations with the Intensity Scale of the Sutter-Eyberg Student Behavior Inventory-Revised and related subscales of the Conners' Teacher Rating Scale-Revised: Long Version (CTRS-R: L). Divergent validity was indicated by nonsignificant correlations between REDSOCS categories and scales on the CTRS-R: L expected not to relate to disruptive classroom behavior. Treatment sensitivity was demonstrated for two of the three primary REDSOCS categories by significant pre to posttreatment changes. This study provides psychometric support for the designation of REDSOCS as an evidence-based assessment procedure for young children. (Contains 3 tables and 2 figures.)